1. Field of the Invention
The present invention relates to an electrical exhaust gas recirculation (EGR) control device for the diesel engine.
2. Description of the Prior Art
Various proposals about EGR control for internal combustion engines, such as the diesel engine, have been made. One example is a disclosure by Japanese Patent Publication SHO 55-151147. This publication discloses a method for controlling the EGR volume in accordance with the engine temperature. A more elaborate EGR control considering the conditions of the engine revolution, load etc. has never been disclosed.
The diesel engine, in which an adiabatic high pressure combustion takes place, generates more various oxides of nitrogen (hereinafter referred to as NOx) under light load than the gasoline engine does; therefore the former needs more of EGR under light load than the latter. To reduce the generation of NOx, it is known that EGR under the condition of low engine revolution, such as idling or near-idling, is effective; and it is desirable that EGR is started at a low engine temperature.
However, if EGR is started under the condition of which the combustion temperature and the exhaust gas temperature are relatively low such as under the idling at the low engine temperature, difficulties are liable to occur such as an increased erosion and abrasion of the cylinder bore or the piston ring due to the condensed water containing the sulphur in the EGR gas. Other problems may include a deterioration of the oil, heavy wear of different parts, deterioration of startability, and a clogging of EGR passage, intake manifold and induction port due to the carbon in the EGR gas, all of which result in a serious drop in the engine durability, reliability and performance.
Thus for the purpose of reducing NOx, EGR must be started before the engine is fully warmed up and EGR must be done under idling (or near-idling), but EGR operation under idling or near-idling under which the combustion temperature and the exhaust gas temperature are low will invite the above-mentioned troubles.